Adds PhysicsPropertyAnimator, which simplifies animation controllers.

See the updated docs for an explanation and example usages. The number of overloaded animateValueForChild methods, and end listener/property setting boilerplate in the controllers were getting annoying, and this is a familiar pattern similar to ViewPropertyAnimator.

Test: atest SystemUITests, also manually tested to ensure no regressions.
Change-Id: Ibca870efda447d79b795c846408f1f6864ce3aa9